Enhanced Localized Energetic-Ion Losses Resulting from Single-Pass Interactions with Alfvén Eigenmodes

We report the first observation of prompt neutral beam-ion losses due to nonresonant scattering induced by toroidal and reversed shear Alfvén eigenmodes in the DIII-D tokamak. The coherent losses are of full energy beam ions expelled from the plasma on their first poloidal orbit. The first-orbit loss mechanism causes enhanced, concentrated losses on the first wall exceeding nominal levels of prompt losses. The loss amplitude scales linearly with the mode amplitude. The data provide a novel and direct measure of the radial excursion or scatter of particles induced by individual modes and may shed light on the mechanism for the scattering of energetic particles in interstellar medium.

Figure 1

Neutral beam injectors on DIII-D. The 30L and 330L neutral beams have identical geometry except for toroidal location. The near-midplane FILD is located at a toroidal angle of 165°. The orbit is the toroidal projection of the first poloidal transit of an ion that is born at the center of the 30L beam near the plasma edge.

Figure 2

Time evolution of (a) 30L (starts from 300 ms) and 330L (starts from 310 ms) power, (b) plasma current and $q_{\min }$, (e) smoothed FILD photomultiplier signal, and (f) toroidal location after first poloidal transit of ions born near the center of the 30L and 330L beams; spectrogram of cross power between vertical and radial ${\mathrm{CO}}_2$ interferometer chords (c) and spectrogram of FILD data (d) in a reversed magnetic shear plasma 146096.

Figure 3

The loss signal rapidly decreases when the corresponding neutral beam (voltage) turns off (consistent with a single poloidal transit period).

Figure 4

(a) Orbits calculated using the SPIRAL code: guiding center (pink) orbit of a 30L beam ion (ionizes at pink square) in the absence of a mode; full (black) and guiding center (yellow) orbit lost to FILD (red triangle) in the presence of an $n=2$, 117 kHz RSAE. (b) Classification of different orbit types in magnetic moment and canonical angular momentum space: lost, trapped, counter passing, co-passing, stagnation, and potato (intersection between trapped and co-passing). The 30L birth profile (black dots) is also shown. The orbits in the purple rectangular region reach the FILD within the detected pitch range at full energy. The start (pink square) and end (red triangle) of the perturbed orbit are marked in (a) and (b). $B_0$ is the on-axis magnetic field; $E=80.4\mathrm{keV}$ is the ion energy; and ${\psi }_{\mathrm{wall}}$ is the poloidal flux at the wall.

Figure 5

Fluctuating FILD loss amplitude ($\Delta F$) vs the mode amplitude for an $n=2$ RSAE at 52 kHz between 325 and 330 ms in discharge 146 096 with edge density profile inserted (upper left). The radial kick ($\zeta$) inferred from Eq. (1) is shown in the color contour.